More than a handful of people are reporting that, after installing the new Windows 10 Build that rolled out to Fast Ring Insider participants late yesterday, they cannot successfully use the supplied product key to activate Windows Enterprise.
When the supplied product key is used, in addition to a "the product key you have entered appears to be a default product key and cannot be used for activation" message, there's also an additional error…
Error code: 0x8007007B Error description: The filename, directory name, or volume label syntax is incorrect.
And answer has been provided in the Windows Insider Program forums:
Open an elevated CMD prompt
Use the following command: slmgr.vbs -ipk PBHCJ-Q2NYD-2PX34-T2TD6-233PK
Microsoft also provides a Microsoft Community article for Insiders that provides additional information on the various ways to activate Windows 10 builds. It’s a link you should keep handy. It includes the above solution, along with about 5 other potential solutions and workaround:
P.S. Some have reported they can no longer open an elevated CMD window in Windows 10 Build 10061, which would negate this solution.